Nuclear magnetic resonance NMR is an important measuring technique with many applications. NMR is based on the precessional properties of tiny magnetic moments of atomic nuclei. In an external magnetic field these nuclei can assume a low-energy state when aligned with the field, or a higher-energy state when aligned against the field. A weak but rapidly alternating magnetic field of appropriate frequency applied by a coil near a specimen will stimulate changes in the orientation of the nuclear magnetic moment relative to the direction of the strong static magnetic field. This results in absorption of energy which is emitted when the nucleus returns to the equilibrium state. The absorption and emission of energy take place at the resonant frequency which is a function of the nuclear species and the static magnetic field strength. A detector picks up the timing and type of the emitted energy from which the chemical makeup of the specimen can be discerned.
Spatial information in NMR data can be obtained by imposing a slight gradient in the static magnetic field. Because the resonance frequency of the signal from the nuclei is proportional to the field, it is possible to derive the position of the nuclei by imposing gradients in different directions and detecting the different frequencies of NMR signals.
An important application of NMR is magnetic resonance imaging (MRI) which is a biomedical technique for depicting tissues within the body. In most MRI medical applications proton NMR signals are used to make anatomical images of biological structures. Three-dimensional reconstruction of images can be achieved using gradients by employing algorithms similar to those in computed X-ray tomography. However, MRI has a number of advantages; it generally provides more contrast between tissues than computed X-ray tomography, and it does this without exposing a patient to any X radiation.
However, conventional MRI also has a number of drawbacks, which include a relatively long data acquisition time (on the order of a minute). As a result, MRI images are subject to motional artifact caused by physiological motions such as heartbeat, blood flow and breathing or voluntary movements of patients. Acquisition time in conventional MRI is long because it requires repeated data acquisitions to acquire an image. In MRI magnetic field gradients allows one to induce spatially and linearly varying frequencies of the NMR signal along the "readout" direction. This produces a spatial spectrum whose Fourier transform is expressed in a time-varying MRI signal. In conventional MRI this signal is seen as an echo whose peak corresponds to the occurrence of coherent phase of the spins along the readout direction. In order to obtain a two-dimensional image, one typically repeats the data acquisition many times to obtain echoes which are identical except for a change in coherence along the "phase encoded" direction. While the actual number and timing of these acquisitions vary greatly, an average image would require 128 data acquisitions each taken 1/2 second apart, requiring 1 minute for total data collection time.
One improvement on conventional MRI is known as fast imaging. In fast imaging techniques the delay between acquisitions is shortened significantly so that the multiple data acquisitions can be obtained in one to ten seconds. Example of fast imaging are two techniques known as GRASS and FLASH.
Another improvement in conventional MRI is known as single-shot imaging. Rather than repeat the data acquisition many times, single-shot techniques allow one to obtain similar copies of an echo in rapid succession in one data collection by the repeated, periodic reversal of the sign of the readout field gradient. This gradient reversal continually "unwraps" the phase changes previously induced, and thus gives repeated coherent echoes. This technique, generally known as Echo-Planar Imaging (EPI) permits the collection of an entire two dimensional data set in a single experiment. This allows rapid collection of data for time-evolution studies and fast screening as well as greatly decreasing motion artifact in the "phase encoded" direction.
The difference between EPI and conventional spin-echo imaging is often illustrated by describing the path traversal in k-space, which is the Fourier representation of physical space as expressed by spatially dependent NMR frequencies. In order to obtain an image, one must sample a two-dimensional k-space "completely." In practice, discrete finite data sampling forces one to limit spatial resolution and spatial boundaries. For further discussion of EPI, see Mansfield, P., J. Phys. C. 10, L55 (1977).
Referring now to FIG. 1, a k trajectory of a conventional spin-echo sequence is shown where one line of k-space is sampled at a time. A typical EPI technique is shown in FIG. 4. Here, the data acquisition covers k-space in one data acquisition. The rapid reversal of the readout gradient is expressed in the reversal of the trajectory along k.sub.x (the x-axis of k-space). If one removes the phase encoding gradients from an EPI experiment, one obtains an echo for each pass through the origin of k space. In FIG. 1 and 4 arrows indicate the direction of the trajectory as determined by the sign of the field gradients during readout. In EPI the addition of small amounts of a magnetic field gradient in an orthogonal direction cause each gradient echo to have the spatial information encoded for this second direction. It will be appreciated that, in both conventional MRI and EPI, the two-dimensional data matrix in FIGS. 1 and 4 is the two-dimensional Fourier transform of the spatial distribution of the detected signal. This matrix can then be transformed to obtain the desired image.
In EPI the image can be generated in much less time than in conventional MRI, e.g., data acquisition periods may be approximately 32-100 milliseconds. Unfortunately, EPI also has a number of disadvantages. It requires specialized hardware which is implemented on only a small percentage of clinical MRI systems. This is because inherent in EPI is the rapid oscillatory switching of a magnetic field gradient between positive and negative values to produce a train of gradient echoes of the MR signal which are detected and parsed into the two-dimensional k-space matrix. This change in readout field gradient over time ("slew rate") is limited by the quality of the hardware which produces the necessary field gradients. Also, the fact that each echo in an EPI data set is a gradient recalled echo rather than a spin echo leads to enhanced artifacts from chemical shift and poor field inhomogeneity.
Thus, it would be desirable to provide an NMR technique which can generate multiple spin echoes in a single data acquisition. It would also be desirable to provide an NMR technique which can be used to produce MRI images in less time than conventional MRI techniques. To this end it would be desirable to provide such a technique which can provide single-shot MRI imaging, thereby decreasing motion artifact in the "phase encoded" direction which often results from multiple data acquisitions. It would also be desirable to provide a single-shot MRI technique which avoids the constant reversal of the readout gradient to permit single-shot MRI imaging in conventional clinical MRI equipment. Further, it would be desirable to provide such a system which avoids chemical shift artifacts as well as artifacts due to magnetic field inhomogeneity which can degrade the MRI images. In addition, it would be desirable to provide these features in a system which permits the utilization of conventional MRI equipment without significant reconfiguration or added cost to the apparatus.